The advancement of research into effective medical treatments that counteract the toxic effects of chemical warfare nerve agent (CWNA) poisoning is heavily dependent on the use of animal models that accurately predict toxicity in humans. Rodents, which are commonly used as small animal models in toxicity research, offer the main benefit of relatively inexpensive in vivo screening of potential countermeasures compared to larger animals and are commonly used in medical chemical defense studies. However, the presence of plasma carboxylesterase activity in rats and mice confounds toxicity reports due to the role that the enzyme has in the detoxification of organophosphorous (OP) compounds. The recently developed carboxylesterase knockout (CaE KO) mouse model may more closely resemble human OP toxicity compared to wildtype mice. However, a comprehensive characterization of the acute and chronic toxic effects of CWNA exposure and medical countermeasures in this innovative animal model is still lacking. A hallmark of CWNA exposure is the onset of status epilepticus (SE), a prolonged seizure that, if uncontrolled, leads to benzodiazepine-resistant SE, and can lead to extensive neuronal damage, cognitive impairment, and epilepsy in animal models. In civilian populations, delayed anticonvulsant treatment is highly likely and currently there are no drugs approved for the delayed treatment of CWNA-induced SE. This project will determine the validity of using the CaE KO mouse as a model to evaluate treatment efficacy against benzodiazepine-resistant OP-induced seizures. The primary objectives are to: 1) characterize the efficacy of the treatment model using the antimuscarinic atropine sulfate and the acetylcholinesterase reactivator HI-6 to increase survival and the anticonvulsant midazolam in the CaE KO mouse model of soman (GD)-exposure when the midazolam treatment is delayed (Specific Aim 1); 2) validate the CaE KO mouse as a model to screen innovative delayed treatments by testing medical countermeasures that are effective in reducing seizure activity and neuropathology in a rat model of GD toxicity (Specific Aim 2); and 3) evaluate in the CaE KO mouse the efficacy of a novel delayed treatment against the acute and long-term effects of GD exposure (Specific Aim 2). This study provides the quantitative characterization of the effectiveness of delayed midazolam treatment in a mouse model that more closely models human OP toxicity, so that the outcome (i.e. seizure activity, behavioral consequences, epileptogenesis and neuropathology) of delayed midazolam can be compared with the addition of novel adjunct medical countermeasures.